A so-called ridge waveguide type semiconductor laser has a striped ridge extending in the resonator length direction, which is produced by etching and thereby removing a portion of an upper cladding layer which is disposed on an active layer. The characteristics of this ridge waveguide type laser largely depend on the distance from the active layer to a light absorption region comprising a burying layer at both sides of the ridge, namely, the thickness of the upper cladding layer which is left at both sides of the ridge through the formation of the ridge by etching. In order to produce a ridge waveguide type semiconductor laser having superior characteristics with improved reproducibility, production methods employing an etching stopper layer have been developed.
FIG. 9 is a cross-sectional view showing a prior art ridge waveguide type semiconductor laser device which is produced employing an etching stopper layer and described in Japanese Journal of Applied Physics Vol. 25, No.6, June, 1986, pp. L498-L500. In FIG. 9, reference numeral 1 designates a substrate comprising n-type GaAs. A lower cladding layer 2 comprising n-type Al.sub.0.5 Ga.sub.0.5 As is disposed on the substrate 1. An active layer 3 having a multi-quantum well structure formed of alternating GaAs layers and Al.sub.0.2 Ga.sub.0.8 As layers is disposed on the lower cladding layer 2. A first upper cladding layer 4 comprising p-type Al.sub.0.5 Ga.sub.0.5 As is disposed on the active layer 3. An etching stopper layer 5b comprising p-type Al.sub.0.3 Ga.sub.0.7 As, a selective etching layer 6 comprising p-type Al.sub.0.5 Ga.sub.0.5 As, a second upper cladding layer 7 comprising p-type Al.sub.0.3 Ga.sub.0.7 As and a cap layer 8 comprising p-type GaAs are laminated successively on the cladding layer 4, resulting in a ridge configuration 9. A current confinement layer 10 comprising n-type GaAs is disposed on the first upper cladding layer 4 at both sides of the ridge part 9 so burying the ridge part 9. Reference numeral 11 designates a Zn diffusion layer into which Zn as p-type dopant is diffused. This Zn diffusion layer 11 is provided for reducing ohmic contact resistance between a p-side electrode which is later formed on the cap layer 8 and the current confinement layer 10, and the cap layer 8. Here, an n-side electrode is disposed on the rear surface of the substrate 1.
Here, the thickness of the first upper cladding layer 4 is 0.3 microns and the thickness of the etching stopper layer 5b is 0.1 micron.
A description is given of the operation.
When a laser is to be operated, a voltage is applied to between the p-side electrode and the n-side electrode, thereby electrons are injected from the n-type substrate 1 and holes are injected from the p-type cap layer 8. The injected holes are confined and flow through a central part of the laser element by the n-type GaAs current confinement layer 10. Thus, electrons and holes are recombined efficiently in the active layer 3 just under the ridge 9 and light of a wavelength equivalent to the energy band gap of the active layer 3 is generated. In this prior art laser, the wavelength of the generated light is about 760 nanometers (nm). The light generated just under the ridge 9 has a tendency to broaden parallel to the active layer 3 but the light is confined in a region just under the ridge 9 due to an effective refractive index difference in the direction parallel to the active layer 3 by light absorption in the current confinement layer 10 in the vicinity of the active layer. This light confinement effect depends on the thickness of the first cladding layer 4 between the active layer 3 and the current confinement layer 10, and when the thickness is 0.2 to 0.3 microns, the best characteristic is exhibited. On the contrary, the light broadening in a direction perpendicular to the active layer 3 is confined to some extent in the active layer 3 by a refractive index difference due to a doublehereto structure, and the other light exits through the sides of the lower cladding layer 2 and the upper cladding layer 4.
A description is given of a method of producing this laser device, especially a method of forming the ridge part 9. FIGS. 11(a) to 11(c) are views showing a formation method of the ridge part 9 in the semiconductor laser device illustrated in FIG. 9. In the figures, the same reference numerals as those of FIG. 9 designate the same or corresponding parts and reference numeral 12 designates a SiO.sub.2 film and reference numeral 13 designates a photoresist film.
First, as illustrated in FIG. 11(a), epitaxial growth is carried out by MOCVD (Metalorganic Chemical Vapor Deposition) to produce successively the lower cladding layer 2, the active layer 3, the first cladding layer 4, the etching stopper layer 5b, the selective etching layer 6, the second upper cladding layer 7 and the cap layer 8, on the substrate 1. Secondly, after the SiO.sub.2 film 12 is formed on the cap layer 8, a photoresist film 13 is formed on the SiO.sub.2 film 12, and a double-layer mask structure comprising the photoresist film 13 and the SiO.sub.2 film 12 having a width corresponding to the width of the ridge 9, is formed employing a photolithography technique and an etching technique. Thereafter, as illustrated in FIG. 11(b), etching is carried out roughly by sputtering employing Ar gas until the thickness t of the selective etching layer 6 outside the ridge becomes 0.3 microns or less. When a wafer is immersed in boiling hydrochloric acid after removing the resist film 13, as illustrated in FIG. 11(c), only the selective etching layer 6 is selectively etched and the etching is stopped by the etching stopper layer 5b. Accordingly, the above described light confinement characteristic confining the light broadening in the direction parallel to the active layer 3 is always determined by the thickness of the first upper cladding layer 4 and thus the reproducibility during production can be significantly improved.
Since boiling hydrochloric acid can etch an Al.sub.x Ga.sub.1-x As layer having a composition ratio X of more than 0.4 but cannot etch an Al.sub.x Ga.sub.1-x As layer having a composition ratio x of 0.4 or less, the composition ratio x of the etching stopper layer 5b is set to 0.4 or less. That is, the etching stopper layer 5b comprises a material having a refractive index higher than that of the upper cladding layer 4 and the lower cladding layer 2. Further, in a completed laser device illustrated in FIG. 9, in order to prevent current broadening in the etching stopper layer 5b, the etching stopper layer 5b outside the ridge is removed by further selective etching.
Since in this prior art device, the etching stopper layer 5b comprising Al.sub.0.3 Ga.sub.0.7 As having a refractive index higher than that of the lower and upper cladding layers 2 and 4 comprising Al.sub.0.5 Ga.sub.0.5 As are disposed on the active layer 3, the proportion of the light confined in the active layer 3 is less than that in a case where the etching stopper layer 5b is absent, because light is close to an interface with a material having a higher refractive index. Because there is generally a tendency that the current value of starting laser oscillation, i.e., threshold current, increases with a decrease of the light confinement coefficient in the active layer of a semiconductor laser device, the etching stopper layer 5b in this prior art device would unfavorably increase the threshold current to a higher value than that of a device with no etching stopper layer. Further, since this etching stopper layer 5b unfavorably affects the light broadening distribution in the direction perpendicular to the active layer, the half value whole angle .theta..perp. of the laser beam broadening in the perpendicular direction may occasionally exhibit a value different from that in a device with no etching stopper layer 5.
In a ridge waveguide type semiconductor laser oscillating at a wavelength of 980 nm, described in ELECTRONICS LETTERS Oct. 24, 1991 Vol. 27 No.22, pp. 2032-2033, an Al.sub.0.22 Ga.sub.0.78 As layer is employed as a cladding layer, i.e., a layer corresponding to the first upper cladding layer 4 and the selective etching layer 6, an AlAs layer is employed as a layer corresponding to the etching stopper layer 5b, and a mixture of succinic acid in an aqueous solution and hydrogen peroxide is employed as an etchant for selective etching in the above described prior art device. Here, an etchant consisting of succinic acid in an aqueous solution and hydrogen peroxide etches Al.sub.0.22 Ga.sub.0.78 As and does not etch AlAs, which results in an improvement in reproducibility of the ridge formation of a ridge waveguide type semiconductor laser. In the semiconductor laser device produced according to this method, the etching stopper layer which is disposed on the active layer of the ridge part has a refractive index lower than that of the cladding layer, and the insertion of this layer avoids the problems that laser threshold current unfavorably increases and that the beam broadening angle in the perpendicular direction should unfavorably varies, with relative to a device employing no etching stopper layer.
AlGaAs lasers are now often used generally for an optical disc device, and in such a case, the oscillation wavelength is fixed below 830 nm (converted into energy, 1.50 eV). On the other hand, it is necessary to design a semiconductor laser device so that carriers injected into an active layer do not overflow into cladding layers sandwiching the active layer, and in an AlGaAs laser, an active layer must be put between cladding layers having an energy band gap which is larger by 0.4 eV or more than the energy equivalent of the oscillation wavelength of the active layer. Therefore, when oscillation wavelength is fixed below 830 nm as described above, an AlGaAs layer having an energy band gap of 1.90 eV or more must be employed as a cladding layer.
The relation between AlAs composition ratio of Al.sub.x Ga.sub.1-x As and the energy band gap Eg can be represented approximately as follows: EQU Eg=1.424+1.247x+1.147*(x-0.45).sup.2 (0&lt;x&lt;0.45) (1) EQU Eg=1.900-0.125x+0.143x.sup.2 (0.45&lt;x&lt;1) (2) (2)
Judging from the formula (1), it is necessary to restrict x.gtoreq.0.38 in order to obtain a band gap energy of 1.90 eV or more. On the other hand, since the variation of Eg due to the increase of x decreases when x exceeds 0.45 as represented by the formula (2), a larger carrier confinement effect cannot be expected. Further, with an increase in x, the refractive index monotonically decreases and the light confinement coefficient of an active layer also monotonically increases. As a result, if x is set too high, high optical density during high power output operation unfavorably deteriorates the laser facet. Therefore, in the AlGaAs laser oscillating at a wavelength below 830 nm, it is desirable for the composition ratio x of the AlGaAs cladding layer to be set around 0.5 (preferably 0.38 to 0.6).
Therefore, when a combination of a cladding layer and an etching stopper layer which is shown in the above described ELECTRONICS LETTERS article is applied to the AlGaAs laser oscillating at a wavelength below 830 nm, a desirable laser structure cannot be obtained.
FIG. 10 is a view showing a semiconductor laser device described in Japanese Patent Published Application No. 3-49289. In FIG. 10, reference numeral 31 designates a substrate comprising n-type GaAs. A lower cladding layer 32 comprising n-type Al.sub.0.5 Ga.sub.0.5 As is disposed on the substrate 1. An Al.sub.0.15 Ga.sub.0.85 As active layer 33 is disposed on the lower cladding layer 32 and a first upper cladding layer 34 comprising p-type Al.sub.0.5 Ga.sub.0.5 As is disposed on the active layer 33. An etching stopper layer 35 comprising p-type Zn.sub.x Cd.sub.1-x S.sub.y Se.sub.1-y is disposed on the first upper cladding layer 34. A second upper cladding layer 36 comprising p-type Al.sub.0.5 Ga.sub.0.5 As is disposed on the etching stopper layer 35 and is formed in a ridge configuration. A current confinement layer 40 comprising n-type GaAs is disposed on the etching stopper layer 35 at both sides of the ridge burying the ridge. A p-type GaAs contact layer 37 is disposed on the ridge and on the current confinement layer 40. Here, an n-side electrode and a p-side electrode are disposed respectively on the rear surface of the substrate 31 and on the contact layer 37.
In this prior art method of forming a ridge for a ridge waveguide type semiconductor laser device, a mixture of tartaric acid in a aqueous solution and hydrogen peroxide is employed as an etchant for selective etching. Namely, the characteristic that this etchant etches Al.sub.0.5 Ga.sub.0.5 As and does not etch Zn.sub.x Cd.sub.1-x S.sub.y Se.sub.1-y, can enhance the reproducibility in the ridge formation of the ridge waveguide type semiconductor laser device. In this laser structure, since Zn.sub.x Cd.sub.1-x S.sub.y Se.sub.1-y employed as the etching stopper layer has a refractive index lower than that of Al.sub.0.5 Ga.sub.0.5 As, the insertion of the etching stopper layer in the above described prior art avoids some problems such as laser threshold unfavorably increasing and beam broadening angle in the perpendicular direction unfavorably varying, with relative to a device employing no etching stopper layer. By employing Al.sub.0.5 Ga.sub.0.5 As for the cladding layer, a desirable AlGaAs laser structure is obtained for oscillating at a wavelength below 830 nm.
In this prior art device, however, since Zn.sub.x Cd.sub.1-x S.sub.y Se.sub.1-y , a II-VI group semiconductor, is employed as the etching stopper layer, an etching stopper layer with good crystallinity cannot be obtained. Difficulties in the production method, i.e., switchings of growth conditions and raw gases, complicate continuous crystal growth on AlGaAs so that it is necessary to control the composition of the etching stopper layer precisely for satisfy lattice matching.